A polymer electrolyte fuel cell (PEFC) is a fuel cell having a form in which: a solid polymer electrolyte is sandwiched between an anode and a cathode; a fuel is supplied to the anode; and oxygen or air is supplied to the cathode, whereby oxygen is reduced at the cathode to produce electricity. As the fuel, hydrogen gas or methanol and the like is mainly used. To enhance a reaction rate in the PEFC and to enhance the energy conversion efficiency of the PEFC, a layer containing a catalyst has been conventionally provided on the surface of a cathode or the surface of an anode of the fuel cell.
As such a catalyst, noble metals have been generally used, and, among the noble metals, platinum having high activity have been mainly used. In order to expand the application of the PEFC, the cost of the catalyst has been trially reduced, and particularly an inexpensive oxygen reduction catalyst containing no platinum and used for the cathode has been trially provided.
Meanwhile, the cathode of the PEFC is placed under a highly acidic-oxidizing atmosphere, and has a higher potential, whereby a material which is stable under the operating environment of the PEFC is extremely limited. In this environment, even platinum which is particularly stable among noble metals has been known to be oxidized and dissolved during long-term use, to cause deteriorated activity. This makes it necessary to use a large amount of noble metal for the cathode also from the viewpoint of maintaining the power generation performance of the PEFC, which has a big problem in terms of both cost and resource.
In order to solve the above-mentioned problems, specifically, from the viewpoint of the expansion of the application of the PEFC, particularly cost reduction and the like, a non-platinum-based oxygen reduction catalyst having high catalyst activity and high durability under the operating environment of the PEFC has been required.
Since metallic sulfide has a small band gap, and exhibits similar conductivity as that of metal, the metallic sulfide is used as a photocatalyst or an electrode catalyst involving an oxygen reduction reaction.
For example, Patent Literature 1 reports that, in a ternary chalcogenide catalyst containing Mo, Ru, and S elements, the ratio of coordination numbers between different elements: ((coordination number between transition metal element-sulfur)/(coordination number between transition metal element-sulfur-oxygen)) is related to the oxygen reduction characteristics of the catalyst.
Non Patent Literature 1 reports oxygen reduction catalysts of cobalt sulfides having different Co/S composition ratios, oxygen reduction catalysts in which cobalt sulfide is doped with transition metal, and synthesis methods thereof.
Non Patent Literature 2 reports oxygen reduction catalysts of cobalt sulfides having different Co/S composition ratios, and synthesis methods thereof.
However, in Patent Literature 1, Ru which is noble metal is used for a catalyst, and is not preferable in terms of cost. The ratio of the coordination number of one element to that of the other is not information on the surface of the catalyst in which the oxygen reduction reaction occurs but a bulk analysis result in the whole catalyst. Co3S4 described in Non Patent Literature 1 originally has oxygen reduction catalyst performance lower than that of CoS2. Non Patent Literature 2 shows that an oxygen reduction activity behavior varies with the Co/S composition ratio of cobalt sulfide, but it does not show an activity behavior related to the composition of a catalyst surface in which an oxygen reduction reaction occurs. Non Patent Literature 2 does not describe cobalt sulfide having a CoS hexagonal structure. Conventional cobalt sulfide has insufficient oxygen reduction catalyst performance, whereby development of a catalyst having higher performance has been desired.